JP2005117308A - Surface acoustic wave device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve a problem in which a conventional surface acoustic wave device which uses a piezoelectric substrate deteriorates in temperature characteristics when obtaining a wide specific band width. <P>SOLUTION: This surface acoustic wave device comprises a dielectric substrate 11, a piezoelectric thin film 13 provided on the top surface of the dielectric substrate 11, and an electrode pattern 14 on the piezoelectric thin film 13. As a result, the surface acoustic wave device having an electromechanical coupling coefficient nearly equal to that of piezoelectric single crystal and excellent temperature characteristics is obtained. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a surface acoustic wave device particularly used for a wireless device, a mobile phone and the like.

  Conventional surface acoustic wave devices have been obtained by forming an electrode pattern such as a comb-shaped electrode pattern on a substrate in which a piezoelectric thin film is formed on a piezoelectric substrate or a non-piezoelectric substrate.

  In the case of a piezoelectric thin film, a structure in which an electrode pattern 102 is provided on a glass substrate 101 as shown in FIG. 7 and a zinc oxide thin film 103 and a counter electrode 104 are provided thereon has been often used.

As prior art document information related to the invention of this application, for example, Patent Document 1 is known.
JP-A-61-92022

  However, in order to obtain filter characteristics with a wide specific bandwidth, a piezoelectric material having a high electromechanical coupling coefficient is required, but a sufficient electromechanical coupling coefficient cannot be obtained with a conventional piezoelectric thin film. For this reason, a single crystal material having a large electromechanical coupling coefficient such as 36 ° Y-cut lithium tantalate is often used. In this case, however, the temperature coefficient becomes relatively large at about −35 ppm / ° C. Had.

  The present invention solves the above-described conventional problems, and an object of the present invention is to provide a surface acoustic wave device having excellent temperature characteristics and a wide specific bandwidth.

  In order to achieve the above object, the present invention has the following configuration.

  According to the first aspect of the present invention, there is provided a dielectric substrate, a piezoelectric thin film provided on the upper surface of the dielectric substrate, and an electrode pattern provided on the piezoelectric thin film. The effect of obtaining a surface acoustic wave device having an electromechanical coupling coefficient comparable to that of a crystal and good temperature characteristics can be obtained.

  The invention according to claim 2 of the present invention is the surface acoustic wave device according to claim 1 in which a piezoelectric thin film is epitaxially grown, and the surface acoustic wave device having excellent characteristics can be realized on various substrates.

  The invention according to claim 3 of the present invention is the surface acoustic wave device according to claim 2 in which the piezoelectric thin film is a lithium tantalate film, and a surface acoustic wave device having a wide band can be realized.

  The invention according to claim 4 of the present invention is the surface acoustic wave device according to claim 2 in which the piezoelectric thin film is a lithium niobate film. Since the coupling coefficient is large, a surface acoustic wave device having a wide band can be realized. .

  The invention according to claim 5 of the present invention is the surface acoustic wave device according to claim 1 in which a conductive layer is provided between the dielectric substrate and the piezoelectric thin film, and a surface acoustic wave device having excellent temperature characteristics can be realized. .

  The invention according to claim 6 of the present invention is the surface acoustic wave device according to claim 5, wherein the dielectric substrate and the piezoelectric thin film are made of the same component, and can realize an optimum parameter for desired device characteristics. it can.

  The invention according to claim 7 of the present invention is the surface acoustic wave device according to claim 6, wherein the dielectric substrate and the piezoelectric thin film are made of lithium tantalate. Since the coupling coefficient is large, the surface acoustic wave device having a wide band is used. realizable.

  The invention according to claim 8 of the present invention is the surface acoustic wave device according to claim 6, wherein the dielectric substrate and the piezoelectric thin film are made of lithium niobate. Since the coupling coefficient is large, the surface acoustic wave device having a wide band is used. realizable.

  The invention according to claim 9 of the present invention is the surface acoustic wave device according to claim 1, wherein the dielectric substrate and the piezoelectric thin film are composed of different components, and the filter characteristics that could not be obtained with the conventional single crystal substrate Can be realized.

  The invention according to claim 10 of the present invention is the surface acoustic wave device according to claim 9 in which the dielectric substrate is quartz, and can achieve excellent temperature characteristics and wide band filter characteristics.

  The invention according to claim 11 of the present invention is the surface acoustic wave device according to claim 10 in which the piezoelectric thin film is a lithium tantalate film, and a surface acoustic wave device having a wide band can be realized.

  The invention according to claim 12 of the present invention is the surface acoustic wave device according to claim 10 in which the piezoelectric thin film is a lithium niobate film, and a surface acoustic wave device having a wide band can be realized.

  A thirteenth aspect of the present invention is the surface acoustic wave device according to the ninth aspect in which a conductive layer is provided between the dielectric substrate and the piezoelectric thin film, and the temperature characteristics can be improved.

  The invention according to claim 14 of the present invention is the surface acoustic wave device according to claim 13, wherein the conductive layer is a metal containing at least one of the metal elements constituting the piezoelectric thin film, and the elastic surface has excellent temperature characteristics. Wave device can be realized.

  The invention according to claim 15 of the present invention is the surface acoustic wave device according to claim 1, wherein the surface acoustic wave propagation velocity of the dielectric substrate is faster than the surface acoustic wave propagation velocity of the piezoelectric thin film, and the high frequency The surface acoustic wave device can be easily realized.

  The invention according to claim 16 of the present invention is the surface acoustic wave device according to claim 15, wherein the dielectric substrate is a single crystal of sapphire, silicon carbide, or diamond or a thin film formed on the substrate. In addition, a surface acoustic wave device having a high frequency can be easily manufactured.

  The invention according to claim 17 of the present invention is the surface acoustic wave device according to claim 16 in which the piezoelectric thin film is a lithium tantalate film, and a surface acoustic wave device having a wide band can be easily realized.

  The invention according to claim 18 of the present invention is the surface acoustic wave device according to claim 16, wherein the piezoelectric thin film is a lithium niobate film, and a surface acoustic wave device having a wide band can be easily realized.

  According to a nineteenth aspect of the present invention, there is provided a semiconductor substrate, a piezoelectric thin film provided on the upper surface of the semiconductor substrate, and an electrode pattern provided on the piezoelectric thin film. Thus, it is possible to obtain a surface acoustic wave device having an electromechanical coupling coefficient of

  The invention according to claim 20 of the present invention is the surface acoustic wave device according to claim 19 in which a piezoelectric thin film is epitaxially grown, and a surface acoustic wave device having sufficient piezoelectric characteristics and bottom loss can be realized.

  The invention according to claim 21 of the present invention is the surface acoustic wave device according to claim 20, wherein the piezoelectric thin film is a lithium tantalate film, and a surface acoustic wave device having a wide band can be realized.

  The invention according to claim 22 of the present invention is the surface acoustic wave device according to claim 20, wherein the piezoelectric thin film is a lithium niobate film, and a surface acoustic wave device having a wide band can be realized.

  The invention according to claim 23 of the present invention is the surface acoustic wave device according to claim 19 in which a conductive layer is provided between the semiconductor substrate and the piezoelectric thin film, and a surface acoustic wave device having excellent temperature characteristics can be realized.

  The invention according to claim 24 of the present invention is the surface acoustic wave device according to claim 23, wherein the conductive layer is a metal containing at least one of the metal elements constituting the piezoelectric thin film, and has a band with excellent temperature characteristics. A wide surface acoustic wave device can be realized.

  The invention according to claim 25 of the present invention is the surface acoustic wave device according to claim 19, wherein the semiconductor substrate is a silicon substrate, and a composite surface acoustic wave device can be realized.

  The invention according to claim 26 of the present invention is the surface acoustic wave device according to any one of claims 5, 13, and 23, wherein a part of the electrode pattern and the conductive layer are electrically connected, and the temperature characteristics are An excellent surface acoustic wave device can be realized.

  A twenty-seventh aspect of the present invention is the surface acoustic wave device according to the twenty-sixth aspect, wherein the surface acoustic wave device is electrically connected through a hole provided in a part of the piezoelectric thin film, and the frequency characteristics of the surface acoustic wave device are stabilized. Therefore, it is possible to realize a surface acoustic wave device that can suppress generation of unnecessary ripples.

  The invention according to claim 28 of the present invention is the surface acoustic wave device according to any one of claims 1 or 19, wherein a portion of the piezoelectric thin film other than the electrode pattern region is removed. A surface acoustic wave device that can prevent deterioration can be realized.

  The invention according to claim 29 of the present invention is the surface acoustic wave device according to any one of claims 1 or 19, wherein the effective electromechanical coupling coefficient of the piezoelectric thin film is 5% or more. A surface wave device can be realized.

  The invention according to claim 30 of the present invention is the surface acoustic wave device according to claim 29, wherein the absolute value of the temperature coefficient of the center frequency is 25 ppm / ° C. or less in the temperature range of 25 ° C. ± 50 ° C. It is possible to realize a surface acoustic wave device with a small frequency fluctuation due to.

  According to a thirty-first aspect of the present invention, the crystal grain size of the piezoelectric thin film is less than 0.7 times or greater than twice the wavelength corresponding to the center frequency. A surface acoustic wave device which is a surface acoustic wave device and has a wide band with a small propagation loss can be realized.

  According to a thirty-second aspect of the present invention, the thickness of the piezoelectric thin film is in the range of 0.5 to 10 times the wavelength corresponding to the center frequency. It is a device, and a surface acoustic wave device having a wide band with a small propagation loss can be realized.

  The surface acoustic wave device of the present invention has an effect that a surface acoustic wave device having an electromechanical coupling coefficient similar to that of a piezoelectric single crystal and having good temperature characteristics can be obtained.

(Embodiment 1)
The invention described in claims 1 to 8 and 26 to 32 of the present invention will be described below using the first embodiment.

  FIG. 1 is a chip cross-sectional view of a surface acoustic wave device according to Embodiment 1 of the present invention, and FIG. 2 is a chip cross-sectional view of a surface acoustic wave device which is another example of Embodiment 1 of the present invention. 3 is a chip cross-sectional view of a surface acoustic wave device which is still another example.

  First, the structure of the surface acoustic wave device of the present invention will be described with reference to FIG. In FIG. 1, a conductive layer 12 made of tantalum is formed on a dielectric substrate 11 made of a 36 ° Y-cut X-propagating lithium tantalate single crystal by sputtering deposition to a thickness of about 0.1 μm. A piezoelectric thin film 13 of lithium tantalate is epitaxially grown and formed on the piezoelectric thin film 13 by MOCVD, and an electrode pattern 14 for 1.9 GHz is formed on the piezoelectric thin film 13. The electrode pattern 14 is a comb-like electrode pattern.

  The conditions for this epitaxial growth are usually that the linear expansion coefficient and the lattice constant of the dielectric substrate 11 and the piezoelectric thin film 13 are close to each other. However, in the first embodiment, the conductive layer 12 is made thin. Thus, it becomes possible to obtain the piezoelectric thin film 13 which is relatively easily epitaxially grown.

  Further, by providing the conductive layer 12 under the electrode pattern 14 via the piezoelectric thin film 13 so that the back surface is short-circuited, the temperature characteristic becomes smaller than the temperature characteristic of the dielectric substrate 11 itself, and the electric machine Since the coupling coefficient of the piezoelectric thin film 13 is epitaxially grown, almost the same characteristics as the dielectric substrate 11 can be realized, so that a broadband surface acoustic wave device with excellent temperature characteristics can be realized.

  When a prototype of a surface acoustic wave device having the above-described configuration is confirmed, a bandwidth almost equal to that produced by only a 36 ° Y-cut X-propagating lithium tantalate single crystal is obtained. It was confirmed that an effective electromechanical coupling coefficient of about 8% was obtained. As a result of investigating the effective electromechanical coupling coefficient of this piezoelectric thin film, it was confirmed that a surface acoustic wave device having a wide bandwidth almost equal to that of a single crystal can be realized if it is 5% or more.

  On the other hand, the temperature coefficient of the conventional surface acoustic wave device was 40 to 45 ppm / ° C., but the product of the present invention was confirmed to be improved to 20 to 25 ppm / ° C.

  As a result of examining the relationship between the surface acoustic wave device relating to this temperature characteristic and an electronic device such as a mobile phone on which the surface acoustic wave device is mounted, the absolute value of the temperature coefficient of the center frequency in the temperature range of 25 ° C. ± 50 ° C. It was found that an electronic device with little characteristic fluctuation in a practical range can be realized by using a surface acoustic wave device having a Pb of 25 ppm / ° C. or less.

  In FIG. 1, the conductive layer 12 exists between the dielectric substrate 11 and the piezoelectric thin film 13, but the surface acoustic wave device can be configured without the conductive layer 12. For example, when the piezoelectric thin film 13 made of an epitaxial thin film of lithium tantalate is provided on the lithium tantalate dielectric substrate 11, the surface on which the electrode pattern 14 is provided is made smoother than the surface of the dielectric substrate 11. A surface acoustic wave device with reduced propagation loss can be realized. Further, by controlling the film stress when the piezoelectric thin film 13 is formed, the propagation speed, the electromechanical coupling coefficient, etc. can be finely adjusted, and the optimum parameters for the desired device characteristics can be obtained.

  Further, although MOCVD is used in the first embodiment, there is no particular problem as long as it is a thin film forming method capable of epitaxial growth, and for example, means such as sputtering may be used.

  Furthermore, in Embodiment 1, lithium tantalate is used for the dielectric substrate 11 and the piezoelectric thin film 13, but it has been confirmed that the same effect can be obtained even if lithium niobate is used. In any case, since the coupling coefficient can be increased, a broadband surface acoustic wave device can be realized.

  Next, the configuration of a surface acoustic wave device according to another example of the present invention will be described with reference to FIG. In FIG. 2, the basic configuration is almost the same as in FIG. 1, and the difference is that a hole 15 is formed in a part of the piezoelectric thin film 13, and a part of the electrode pattern 14 and the conductive layer are formed through the hole 15. 12 are electrically connected. By adopting such a configuration, it is possible to stabilize electrical characteristics such as temperature characteristics of the surface acoustic wave device.

  Next, the configuration of a surface acoustic wave device according to another example of the present invention will be described with reference to FIG. In FIG. 3, the basic configuration is almost the same as that in FIG. 1, and the difference is that the piezoelectric thin film 13 is removed except for the region of the electrode pattern 14. By adopting such a configuration, the edge portion of the piezoelectric thin film 13 can be used as an end reflector, so that the surface acoustic wave device can be downsized.

  Further, when the crystal grain size of the piezoelectric thin film 13 becomes substantially equal to the wavelength corresponding to the center frequency, reflection tends to occur at the boundary, and propagation loss increases. Therefore, it is desirable to make the crystal grain size smaller than 0.7 times the wavelength or larger than twice the wavelength.

  In addition, when the thickness of the piezoelectric thin film 13 is 0.5 times or less of the wavelength, a sufficient electromechanical coupling coefficient cannot be obtained, and when the thickness is too thick, sufficient crystallinity cannot be obtained or peeling easily occurs. 0.5 times to 10 times is desirable.

  Further, an insulating film such as silicon dioxide may be further provided on the electrode pattern 14 in order to improve temperature characteristics.

(Embodiment 2)
The invention according to claims 9 to 14 of the present invention will be described below using the second embodiment.

  The difference between the second embodiment and the first embodiment is that, in the first embodiment, the dielectric substrate 21 and the piezoelectric thin film 23 having the same components are used but the second embodiment is different. It is a point using.

  In FIG. 4, a conductive layer 22 made of tantalum is formed on a dielectric substrate 21 made of quartz by sputtering deposition to a thickness of about 0.1 micrometers, and a thickness of about 4 micrometers is formed thereon by MOCVD. Then, a piezoelectric thin film 23 of lithium tantalate is epitaxially grown, and an electrode pattern 24 of 1.9 GHz is formed thereon.

  Although the conditions for epitaxial growth are stricter than in the first embodiment, the combination of the dielectric substrate 21 and the piezoelectric thin film 23 can further improve the temperature characteristics and obtain a temperature coefficient of 15 ppm / ° C. or less. did it.

  In the case of the second embodiment, the conductive layer 22 can be omitted, but it is desirable for better characteristics. The material of the conductive layer 22 is preferably a metal containing at least one of the metal elements constituting the piezoelectric thin film 23. In the second embodiment, tantalum is used. With such a configuration, the crystallinity of the piezoelectric thin film 23 can be increased, and as a result, the temperature characteristics of the surface acoustic wave device can be made excellent.

(Embodiment 3)
Hereinafter, the invention described in claims 15 to 18 of the present invention will be described using the third embodiment.

  The difference between the third embodiment and the first embodiment is that in the first embodiment, the dielectric substrate 31 and the piezoelectric thin film 33 having the same components are used, but in the third embodiment, the dielectric substrate is used. 31 is faster than the surface acoustic wave propagation velocity of the piezoelectric thin film 33.

  In FIG. 5, a conductive layer 32 made of tantalum is formed on a dielectric substrate 31 made of sapphire to a thickness of about 0.1 micrometer by sputter deposition, and then a thickness of about 4 micrometers is formed thereon by MOCVD. A piezoelectric thin film 33 of lithium tantalate is epitaxially grown, and an 1.9 GHz electrode pattern 34 is formed thereon.

  With such a configuration, a surface acoustic wave device having an electromechanical coupling coefficient similar to that of a piezoelectric single crystal and good temperature characteristics can be obtained.

  Even when the frequency is high, the use of sapphire, which is a high sound velocity substrate, for the dielectric substrate 31 eliminates the need for a relatively fine pattern and can stabilize the process.

  In this case, if the film thickness of the piezoelectric thin film 33 is made too large, the surface acoustic wave propagation velocity also decreases.

  In the third embodiment, a sapphire single crystal substrate is used as the dielectric substrate 31, but a piezoelectric thin film 33 such as diamond may be formed on another dielectric substrate 31.

  Further, it has been confirmed that the piezoelectric thin film 33 can obtain the same effect even if it is an epitaxial film of lithium niobate.

(Embodiment 4)
Hereinafter, the invention described in claims 19 to 25 of the present invention will be described using the fourth embodiment.

  The difference between the fourth embodiment and the first embodiment is that the dielectric substrate 11 is used as the substrate in the first embodiment, whereas the semiconductor substrate 41 is used in the fourth embodiment. is there.

  In FIG. 6, a conductive layer 42 made of tantalum is formed on a semiconductor substrate 41 made of silicon by sputtering deposition to a thickness of about 0.1 micrometers, and a thickness of about 4 micrometers is formed thereon by MOCVD. Then, a piezoelectric thin film 43 of lithium tantalate is epitaxially grown, and an electrode pattern 44 of 1.9 GHz is formed thereon.

  By doing so, a surface acoustic wave device having an electromechanical coupling coefficient comparable to that of a piezoelectric single crystal and good temperature characteristics can be obtained.

  In addition, a device in which a semiconductor circuit and a surface acoustic wave device are integrated is possible.

  The surface acoustic wave device according to the present invention has an electromechanical coupling coefficient similar to that of a conventional single crystal substrate, and has an effect that a characteristic with a small temperature coefficient can be obtained. Useful for filters in the video field.

Sectional drawing of the surface acoustic wave device in Embodiment 1 of this invention Cross section Cross section Sectional drawing of the surface acoustic wave device in Embodiment 2 of this invention Sectional drawing of the surface acoustic wave device in Embodiment 3 of this invention Sectional drawing of the surface acoustic wave device in Embodiment 4 of this invention Sectional view of a conventional surface acoustic wave device

Explanation of symbols

11, 21, 31 Dielectric substrate 12, 22, 32, 42 Conductive layer 13, 23, 33, 43 Piezoelectric thin film 14, 24, 34, 44 Electrode pattern 15 Hole 41 Semiconductor substrate

Claims (32)

A surface acoustic wave device comprising a dielectric substrate, a piezoelectric thin film provided on the upper surface of the dielectric substrate, and an electrode pattern on the piezoelectric thin film. 2. The surface acoustic wave device according to claim 1, wherein the piezoelectric thin film is an epitaxially grown film. 3. The surface acoustic wave device according to claim 2, wherein the piezoelectric thin film is a lithium tantalate film. 3. The surface acoustic wave device according to claim 2, wherein the piezoelectric thin film is a lithium niobate film. 2. The surface acoustic wave device according to claim 1, wherein a conductive layer is provided between the dielectric substrate and the piezoelectric thin film. 6. The surface acoustic wave device according to claim 5, wherein the dielectric substrate and the piezoelectric thin film are made of the same component. The surface acoustic wave device according to claim 6, wherein the dielectric substrate and the piezoelectric thin film are lithium tantalate. The surface acoustic wave device according to claim 6, wherein the dielectric substrate and the piezoelectric thin film are made of lithium niobate. 2. The surface acoustic wave device according to claim 1, wherein the dielectric substrate and the piezoelectric thin film are made of different components. The surface acoustic wave device according to claim 9, wherein the dielectric substrate is made of quartz. The surface acoustic wave device according to claim 10, wherein the piezoelectric thin film is a lithium tantalate film. The surface acoustic wave device according to claim 10, wherein the piezoelectric thin film is a lithium niobate film. The surface acoustic wave device according to claim 9, wherein a conductive layer is provided between the dielectric substrate and the piezoelectric thin film. The surface acoustic wave device according to claim 13, wherein the conductive layer is a metal containing at least one of metal elements constituting the piezoelectric thin film. 2. The surface acoustic wave device according to claim 1, wherein the surface acoustic wave propagation speed of the dielectric substrate is faster than the surface acoustic wave propagation speed of the piezoelectric thin film. 16. The surface acoustic wave device according to claim 15, wherein the dielectric substrate is a single crystal of sapphire, silicon carbide, or diamond or a thin film formed on the substrate. The surface acoustic wave device according to claim 16, wherein the piezoelectric thin film is a lithium tantalate film. The surface acoustic wave device according to claim 16, wherein the piezoelectric thin film is a lithium niobate film. A surface acoustic wave device comprising a semiconductor substrate, a piezoelectric thin film provided on the upper surface of the semiconductor substrate, and an electrode pattern on the piezoelectric thin film. The surface acoustic wave device according to claim 19, wherein the piezoelectric thin film is an epitaxially grown film. The surface acoustic wave device according to claim 20, wherein the piezoelectric thin film is a lithium tantalate film. 21. The surface acoustic wave device according to claim 20, wherein the piezoelectric thin film is a lithium niobate film. The surface acoustic wave device according to claim 19, wherein a conductive layer is provided between the semiconductor substrate and the piezoelectric thin film. The surface acoustic wave device according to claim 23, wherein the conductive layer is a metal containing at least one of metal elements constituting the piezoelectric thin film. The surface acoustic wave device according to claim 19, wherein the semiconductor substrate is a silicon substrate. 24. The surface acoustic wave device according to claim 5, wherein a part of the electrode pattern and the conductive layer are electrically connected. 27. The surface acoustic wave device according to claim 26, wherein the surface acoustic wave device is electrically connected through a hole provided in a part of the piezoelectric thin film. The surface acoustic wave device according to claim 1, wherein a portion of the piezoelectric thin film other than the electrode pattern region is removed. The surface acoustic wave device according to claim 1, wherein an effective electromechanical coupling coefficient of the piezoelectric thin film is 5% or more. 30. The surface acoustic wave device according to claim 29, wherein an absolute value of a temperature coefficient of a center frequency is 25 ppm / ° C. or less in a temperature range of 25 ° C. ± 50 ° C. The surface acoustic wave device according to claim 1 or 19, wherein the crystal grain size of the piezoelectric thin film is smaller than 0.7 times or larger than twice the wavelength with respect to the center frequency. The surface acoustic wave device according to claim 1 or 19, wherein the thickness of the piezoelectric thin film is in the range of 0.5 to 10 times the wavelength corresponding to the center frequency.

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